A-c energizing system with static interrupter

ABSTRACT

A system for energizing an AC load from an inverter, or from other single- or multi-phase power sources or combination thereof, includes a series filter comprising a capacitor coupled in series with an inductor. The filter elements are resonant at the normal operating frequency of the inverter. Responsive to a large overload or a fault condition in the system, a virtual short circuit is applied across one of the filter components to detune the filter. Alternatively if a load exceeding a specified value is applied, a clamp detunes the filter. This significantly increases the effective impedance seen by the inverter or other power sources, protecting the system against excess currents.

United States Patent 91 Krauthamer 1 Jan. 16, 1973 [54] A-C ENERGIZINGSYSTEM WITH 3,356,900 12 1967 Kalkner ..317 20 STATIC INTERRUPTER IFOREIGN PATENTS OR APPLICATIONS [75] Inventor: Stanley Krauthamer,Monterey 1,114,243 /1968 Great Britain ..321/l4 Park,Ca11f.

1,131,504 /1968 Great Britain ..3l7/20 [73] Assignee: Borg-WarnerCorporation, Chicago,

111. Primary Examiner-William H. Beha, Jr. [22] Filed: June 18 1971AttorneyDonald W. Banner et a1.

[ pp 154,382 [57] ABSTRACT v I A system for energizing an AC load froman inverter, 1 or from other singleor multi-phase power sources or333/76 combination thereof, includes a series filter compris- [51 Int.Cl. .1102!!! 1/18, H0211 7/14 ing a capacitor coupled in series with aninductor. The 1 1 Field of 3 filter elements are resonant at the normaloperating frequency of the inverter. Responsive to a large over- I loador a fault condition in the system, a virtual short [56] References C tecircuit is applied across one of the filter components to detune thefilter. Alternatively if a load exceeding a UNITED STATES PATENTSspecified value is applied, a clamp detunes the filter. 1,625,464 4 1927Gay ..317/20 x i ignifi an ly increases the effective impedance 1,755,]11 4/1930 seen by the inverter or other power sources, protect-3,436,600 4/1969 ing the system against excess currents. 1,651,02111/1927 3,295,018 12/1966 3 Claims, 11 Drawing Figures 23 24 I03 amp iI02 4b lGb 7 a. m 104 14c 1 m: 86 87 cm, 62

Rectifier Firing Circuit Static Switch 52 PATENTEDJlllllS I975 3.711.759

SHEET 1 [IF F IG. l I I5 Series Filter I0 I l2 l4 l8 To A-O t lnver erWM Load l3 Series Filter 25 I2 I4 l8 To A-C Inverter fiw Load Detector2| 30 7 Freq.Set mg 37 3H6. 3 I5 35 53 Oscillator T Driver Y T 232:: I

T o I 38 36 I4 42 23 I2 46 Parallel t Filter l 47 48 D C Transformer Busl 44 T I 54 50 5| 43 Firing a Static Inventor T Circuit j Switch IStanley Krouthomer L- By Att rrief PATENTEDJIIII I6 I973 DC. InputSystem SHEEI 3 [It 5 Inverter Filter Protective Ci rc u it SystemInverter Filter Protective Circuit System *3 Inverter Fitter LoodProtective Ci rcu it FIGS Inventor Stanley Krouthomer PATENTEDJAN 16 mmSHEET 5 BF 5 lnverrer I92 D-Cin Inverter ZIS FIG.7C

Inventor Sfonley Kruurhomer yuL. J J

Attorney A-C ENERGIZING SYSTEM WITH STATIC INTERRUPTER BACKGROUND OF THEINVENTION Since the early days of the power supply and distributionindustry, there has always been a need for effective circuit breakerarrangements to protect the supply equipment when faults occur on theline or on the load serviced by the equipment. To this end various typesof switches were developed as circuit breakers to interrupt the supplycircuit, either automatically when an overload was sensed, or manuallywhen an attendant noticed an abnormal condition. A disadvantage of mostsuch arrangements is the finite time required for the physicaldisplacement of a switch or other circuit-opening arrangement to protectthe equipment. Thus a primary consideration of the present invention isto provide a protective arrangement for an energy supply system whichdoes not require the physical movement of any component to effect thecircuit protection. Such an arrangement provides circuit protectionwithout the time delays associated with fuses and mechanical circuitinterrupters.

Another important consideration of the present invention is to utilizecomponents already in the circuit, and modify the normal operation to ineffect derive another function that of circuit protection from acomponent employed in the circuit for another purpose.

SUMMARY OF THE INVENTION The present invention is useful in anenergizing system which applies AC energy from an inverter or otherpower source over a filter which includes first and secondseries-coupled reactive components, such as a capacitor coupled inseries with an inductor. These components form a filter circuit which isresonant at the normal operating frequency of the inverter. Circuitmeans is provided, and coupled to one of the reactive filter components,to effect a substantial change in the effective impedance of that onereactive component. In a preferred embodiment this circuit meansincludes a static switch and a very low value resistance. Thisresistance is coupled, upon operation of the static switch, over atransformer to shunt one'of the reactive components in the seriesfilter.

A control circuit is also provided, and connected to regulate operationof the circuit means responsive to the detection of an overloadcondition associated with the inverter. In a preferred embodiment thisis accomplished by sensing the undesired condition, such as a fusefailure, or a current overload in a circuit associated with the inverteror the load. Responsive to the detection of this undesired condition,the static switch is closed to substantially change the impedance of theone reactive component. This removes the resonant condition, and offersa higher impedance at the output side of the inverter. This substantialchange in circuit parameters thus protects the inverter against a shortcircuit or other fault condition.

THE DRAWINGS In the several figures of the drawings like referencenumerals identify like elements, and in the drawings:

FIGS. 1 and 2 are propaedeutic illustrations useful in understandingbasic arrangements of the invention;

showings, and FIG. 7B is a graphical illustration, of ad- 0 ditionalembodiments of the invention.

GENERAL SYSTEM DESCRIPTION FIG. 1 shows a system with DC energy beingsupplied over input line 10, fuse l1, and line 12 to an inverter 13. Theinverter passes output AC energy over line 14, series filter 15, whichincludes a capacitor 16 and an inductor l7, and over an output line 18to any AC load. The first and second reactive components 16 and 17within the filter are sized and connected to be resonant at the normaloperating frequency of the AC output voltage supplied by inverter 13.Accordingly the net effective impedance to the energy flow betweeninverter 13 and output line 18 is very low at the normal operatingfrequency, where the reactance of the capacitor is substantially equalto the reactance of the inductor. Those skilled in the art willappreciate this arrangement can be employed with any AC supply line 14,including systems other than those having an inverter to provide the ACvoltage which is passed to the filter.

In accordance with the present invention, a circuit means shown as relay20 is provided. The relay includes a winding 21 and a contact set 22which, when closed, effectively short circuits capacitor 16. Line 23 andenergizing terminal 24 together represents a control circuit connectedto regulate operation of the relay. That is, a potential is applied toterminal 24 substantially equal to that present on line 12 under normaloperating conditions. Responsive to an overload or fault such that fuse11 blows, the potential on line 12 rapidly changes. The resultant netpotential difference between line 23 and terminal 24 is applied acrossrelay winding 21, actuating the relay and closing contacts of 22 tovirtually short circuit capacitor 16. This is a substantial change inthe effective impedance of the reactive component 16, so that the netreactance of inductor 17 minus that of capacitor 16 is inserted" in thecircuit. Before the operation of the relay 20, the two impedances in thefilter virtually cancelled each other. After relay 20 is actuated, theunbalance between the two reactances is inserted into the circuit toprotect the total system arrangement. Thus a fault in the load, whichwould otherwise tend to draw excess current, is no longer thesubstantial hazard it was if the present invention is employed.

It may be desired to protect the system against slight changes in thecurrent level, changes of a level insufficient to blow fuse 11. FIG. 2shows such an arrangement in which a resistor 25 of very low ohmic valueis inserted in the supply line to inverter 13. Thus a signal indicativeof the current flow is applied over line 26 to a detector circuit 27.This circuit also receives a reference signal over line 28 from apotentiometer 30. Accordingly when the current level signal on line 26exceeds the reference level established by the position of the arm ofpotentiometer 30, detector 27 provides an output signal to actuate relay20 in a well known manner. At this time the series filter is de-tuned bythe short circuiting of capacitor 16, and the inverter and other circuitcomponents are protected against damage by excessive current.

DETAILED DESCRIPTION OF THE INVENTION FIG. 3 depicts a general systemfor regulating the operation of an AC motor 35. As there shown inverter13 receives frequency-controlling pulses over line 36 from a driverstage 37, which in turn receives a series of timing pulses over line 38from an oscillator 40 which has a frequency set adjustment representedby knob 41. By regulating the frequency of the oscillator pulses, thefrequency of alternation of the inverter output voltage is governed, andthus the speed of motor 35 is correspondingly regulated in a well knownmanner. Another filter 42 is shown connected in parallel with outputline 18. Such a parallel-connected filter is sometimes used for rippleattenuation. The details and further explanation of such an arrangementare not necessaryto an understanding or implementation of the presentinvention.

In accordance with the present invention, a static switch 43 is providedto effect a circuit operation analogous to the operation of relay inFIGS. 1 and 2. In its operation static switch 43 reflects a very lowimpedance or resistance over line 44, transformer 45 and line 46 to oneof the reactive components in series filter 15. Energy from the DC bus47 is passed over line 12 to the inverter, and is also passed over line48 and a portion of transformer 45 to the static switch, as will bedescribed more fully hereinafter.

A firing circuit 50 provides a control signal over line 51 to regulateoperation of static switch 43. The firing circuit receives AC energyover line 52, which is rectified within circuit 50. A control inputsignal is received by the firing circuit over line 23 from a fuse 11,shown in this arrangement within inverter 13. Of course, the fuse can bepositioned externally to the inverter, or the signal could come from adetector stage such as that represented by 27 in FIG. 2. After apredetermined time, a relay is actuated within firing circuit 50 toeffect the opening of contact set 53 over the mechanical actuatorrepresented generally by broken line 54, to interrupt energization ofmotor 35. With this perspective of the system as a whole, a moredetailed description of the arrangement shown in FIG. 4 will now be setout.

As there shown the invention is illustrated in connection with athree-phase system. Those skilled in the art will appreciate that theprinciples are also applicable to a single-phase or two-phase system, byremoving two or one of the phase circuits and the attendant circuitryshown in FIG. 4.

As there shown the three-phase output voltage from an inverter or otherAC source is passed over lines 14a, 14b, and 140 toward the load. Theseries filter in line 14a includes capacitor 16a and inductor 17a;filter 16b and 17b is connected in line 14b; and in line 140, a filtercomprising capacitor 160 and inductor 17c is coupled. Transformer 45 hasthree windings 60, 61, and 62 coupled to the respective windings 63, 64and 65. Winding 60 is coupled in parallel with capacitor 16a in thefirst phase circuit, and the other windings 61, 62 are similarly'coupledacross the respective capacitors in the other phase circuits. Of coursethese windings could be coupled across the inductors, instead of acrossthe capacitors. As long as one of the reactive components is shorted,the effective reactance of the other serves to limit the load currentupon a fault or current overload.

The center tap connection of winding 63 is coupled over line 66 to aconnection 67 in the static switch 43. Likewise line 68 extends theconnection from the center tap connection of winding 64 to connection 70in the static switch, and the center tap connection of winding 65 isextended over line 71 to connection 72 in the static switch 43. Diode 73is coupled between bus conductor 12a and one side of winding 63; diodes74 and 75 are similarly coupled between the additional windings 64, 65and the DC bus conductor 12a. Thus when an overload condition occurs,for example on line 14a, a voltage is developed across capacitor 16awhich increases above the normal voltage across this capacitor. Thisincreased voltage is applied over windings 60, 63 of transformer 45 anddiode 73 to the DC bus 12a, 12b. However with the diode connection thisvoltage across capacitor 16a is effectively clamped at the level of themain DC bus voltage across conductors 12a, 12b. Thus the vectordifference in the voltages across reactive components 16a and 17aprovides an effective current-limiting reactance with respect to the ACline.

Static switch 43 includes a half-wave three-phase bridge, with threeSCRs 76, 77 and 78 and three diodes 80, 81 and 82. Each of the SCR's hasits cathode coupled to common conductor 83, and each of the diodes hasits anode coupled to another common eonductor 84. In a single-phasesystem only one set of SCR-diode connections would be provided. Forexample, with only a single series filter including capacitor 16a andinductor 17a, only windings 60 and 63 would be provided, and SCR 78would be coupled to diode 82 as shown. Resistor 85 is provided as a verylow resistance between the common conductors 83 and 84. This is thelevel of the resistance or impedance reflected back over transformer 45to shunt the reactive components in the series filter. In a preferredembodiment this reactance was of a very low ohmic'value, 0.055 ohm. Theresistor was rated at 200 watts because of its position in the circuit.Those skilled in the art will understand that resistive wire or anothersimilar component could be provided in place of the illustrated resistor85.

In firing circuit 50, transformer 86 is connected to pass AC energyreceived over lines 52a, 52b to a rectifier circuit 87 forproviding alow DC voltage across capacitor 88, which is applied between conductors90, 91 to energize the components in the firing circuit. Betweenconductors 90, 91 is a series circuit comprising resistor 92 and diodes93, 94. These components are sized to provide a DC voltage of theappropriate level between conductors 95, 96 when a firing signal isapplied to the gate of SCR 97. SCR 97 has its anode connected toconductor 90,'its cathode connected to conductor and its gate coupledboth to one side of a capacitor 98, and to one side of the secondarywinding 100 of a transformer 101, which also includes a primary winding102. One end of primary winding 102 is coupled to terminal 24, and theother end is coupled over a parallel circuit including resistor 103 anda capacitor 104 to conductor 23, over which the fuse failure,overcurrent, or other fault indication is received.

The other side of secondary winding 100 and of capacitor 98 is coupledto conductor 95. The gate of SCR 97 is also coupled to the cathode ofdiode S, and to one side of resistor 106, the other end of which iscoupled to conductor 91. Relay winding 107 is coupled between conductors95, 96. Accordingly upon firing of SCR 97 this relay winding isenergized to open the contact set 53 (FIG. 3) after a predetermined timeinterval, thus protecting both the switching arrangement and the mainenergy path from severe damage.

Conductor 96 is coupled to the cathode of each of the SCRs 76, 77 and78. The firing circuit is completed to the three gates over threeindividual isolation circuits. Circuit 108 for the gate of SCR 76includes a capacitor 110 coupled in series with resistor 111, andanother resistor 112 coupled in series with diode 113. These two seriescircuitsare coupled in parallel, and the intermediate points are coupledtogether over a conductor 114. Thus any potential applied betweenconductors 95, 96 is translated over circuit 108 and applied between thegate and cathode of SCR 76 to rapidly drive this semiconductor switchon. The gate circuit 115 for the SCR 77 includes components 116-119, andgate circuit 120 for the third SCR 78 includes components 121-124.

In operation, it is assumed that the motor control system is energizedand AC energy is being transferred over conductors 14a, 14b and 140toward the AC motor or any other AC load. At this time AC voltage ispassed over transformer 86 and rectified in circuit 87, to provide a DCoperating potential between conductors 90 and 91 in firing circuit 50.Under normal conditions, when there is no overload or fuse failure, SCR97 remains nonconducting and all of SCRs 76, 77 and 78 arenonconducting.

If there is an overcurrent condition on the AC line, of a levelsufficient to cause the voltage to rise across the capacitors 16a-16cbut not sufficient to blow the fuse, the voltage across capacitors16a-16c is clamped as explained above. The excess voltage is returnedover the circuit including diodes 73-75 to the DC bus 12a, 12b. Thusthere is a certain level of protection afforded by the clampingarrangement even before a fuse failure or other serious fault occurs.

Responsive to the failure of a fuse or an excess overcurrent condition,a signal is provided on line 23 which passes a control pulse signal overtransformer 101 to the gate of SCR 97. SCR 97 is gated on and completesa circuit between conductors 90 and 95, to pass gating pulses over eachof circuits 108, 115, and 120 to the respective gates of SCRs 76-78.These SCRs are fired, and in effect connect low resistance 85 across thewindings 63-65 of transformer 45. This low impedance is reflected acrossthe windings 60-62 to substantially short out capacitors 16a-l6c. Thisremoves the resonant condition of the filter, and the inductors 17a, 17band 17c then protect the output circuit from excess current conditions.

At a time interval determined by the constants of the relay includingwinding 107, such as the setting of a dashpot arrangement, this relayoperates and opens contact set 53 to interrupt the transfer of energytoward the AC load 35. Thus the resistor 85 is prevented fromoverheating. With interruption of the energy transfer, the AC voltage isalso removed from conductors 52a and 52, and firing circuit 50 isde-energized.

The use of static switch 43 and firing circuit 50, in conjunction withtransformer 45, provides very effective short circuit decoupling in thecircuit between the inverter and the AC load. It has been found that useof the transformer 45 itself, together with the diodes 73-75 forclamping back to the DC bus, provides a significant measure ofprotection even without the static switch and firing circuit. Thecurrent limiting achieved between the inverter and the load, employingonly the transformer and diode, insures that an excessive fault currentwill not be transmitted over the inverter-load circuit if there is ashort at the load or some other failure in the system. Thus this currentlimiting feature can be utilized apart from the complete filter detuningaspect of this invention.

FIG. 5 shows a general system arrangement in which a plurality of ACsystems, each including an inverter such as 13 and a series-coupledfilter such as 15, are connected to supply AC energy to any AC load 130.The inverter and filter combination such as 13-15 has already beendescribed in connection with FIGS. 1-3. The protective circuit in systemNo. 1 of FIG. 5 is referenced 131, and may include components such astransformer 45, static switch 43, and firing circuit 50. As has alreadybeen emphasized, protective circuit 131 could include only thetransformer and diodes for clamping the voltage across one of thereactive filter components at the DC bus level.

System No. 2 includes a similar inverter-filter combination. Inverter133 receives DC input energy over line 132, and passes AC energy overline 134 to filter 135, which is coupled over line 138 to the load 130.Protective circuit 137 of system No. 2 is coupled over line 136 tofilter 135. Similarly in system No. 3 DC energy from bus 47 is suppliedover line 142 to inverter 143, which passes AC energy over line 144,through filter 145 to output line 148. Protective circuit 147 is coupledover line 146 to filter 145. The DC input energy can be supplied to theinput lines 12, 132, and 142 from individual batteries, DC generators,or other sources, in lieu of a common bus as indicated. Other variablesof the basic system may be suggested to those skilled in the art.

With the system of FIG. 5 energized and supplying load 130, if onesystem such as No. 1 fails, then it is desirable to decouple system No.1 from systems No. 2 and No. 3. If this is done the fault currentsupplied by systems No. 2 and No. 3 would be sufficiently high todestroy these other operating systems. This is an important use of theprotective circuit arrangement shown generally in FIG. 3 and morespecifically in FIG. 4.

FIG. 6A depicts another arrangement for protecting the inverter-loadenergy transfer circuit, by modifying the normal series filterconfiguration. In accordance with this aspect of this invention, staticswitch circuit 43 is coupled to the common connections or terminals 150,151 and 152 between the series-connected inductor and capacitor in eachof the output circuits. In the arrangement depicted in FIG. 4, if thereis a lock-on" (inverter component failure), or a summing transformerfailure, the load sees an inductive impedance.

This is also true if failure of an inductor in any of theseries-connected filters occurs. Under these fault conditions the loadmust feed a substantial current, at a low power factor, into the fault.However by utilizing the circuit of FIG. 6A, when the inverter fails orlocks on, static switch 43 is immediately triggered on. This firing ofthe static switch essentially shorts terminals 150, 151 and 152together. Thus a three-phase, Y-connected capacitor arrangement ispresented to the three-phase load. This arrangement completely isolatesthe load from any lock-on or inverter component failure, from anyfailure of a summing transformer, or from an inductor failure. The loadthen sees a leading power factor, thus improving the system operation.The system is still operational if one of the capacitors fails.

FIG. 68 illustrates another circuit arrangement which utilizes thecommon terminals 150, 151 and 152 in the series filters. As there shownthe upper windings 60, 61 and 62 of transformer 45 are coupled to thecommon terminals 150, 151 and 152.,With this arrangement current limitis applied to the common connections, instead of across a reactivecomponent in the filter circuit. The arrangement of FIG. 6B can be usedindependently or with the arrangement of FIG. 6A, with both the staticswitch and the firing circuit being connected as depicted previously inFIG. 4.

An alternate method of coupling transformers across common terminals150152 is set out in FIG. 6C. As there shown a three-phase transformer160 includes primary windings 161, 162 and 163, magnetically coupled tothe respective secondary windings 171, 172 and 173. The secondarywindings are coupled through a rectifier bridge 175, which includes sixdiodes 176-181, to the DC bus conductors 12a and 12b. In this way therespective line-to-line voltages, such as the voltage between terminals150 and 151, are clamped back to the DC bus level. By preventing theseline-toline voltages from rising, current limit is achieved with thearrangement shown in FIG. 6C.

FIG. 7A indicates another circuit for modifying the series-connectedfilter including reactive components 17a and 16a. As shown one outputconnection from inverter 183 is extended over line 184, inductor 17a,terminals 185, 186, capacitor 16a, terminal 187 and output conductor 198toward the load. The other output connection from inverter 183 iscoupled over conductor 188, terminal 190 and output conductor 199 tocomplete the energizing circuit. Capacitor 42a and inductor 42b comprisea parallel filter, as described generally in connection with FIG 3.

The arrangement of FIG. 7A is useful as a transient suppressor foruninterruptablc power supply (UPS) systems. In accordance with thisaspect of the invention a by-pass circuit 191 is provided aroundinductor 17a. This bypass circuit includes a resistor 192 coupled inseries with a static switch 193. As shown the static switch comprises apair of parallel-connected SCRs 194, 195. As gating signals are receivedover conductors 196, 197. these SCRs are gated on. This completesashunting circuit so that resistor 192 is essentially connected toby-pass inductor 17a. Those skilled in the art will appreciate thatstaticswitch 193 can be replaced by a thyristor of the type generallytermed a Triac, to provide conduction in both directions from a singlesemiconductor switch.

Another static switch 200, including another pair of back-to-back SCRs201, 202, is coupled in series with a resistor 203. This circuit iscoupled between terminals 186 and 190, so that upon actuation of thestatic switch 200, resistor 203 is placed in the circuit between outputconductor 199 and the common terminal 186 in the filter 17a, 16a.

When the load is initially applied across conductors 198, 199, the firststatic switch 193 is fired by applying gating signals over conductors196, 197. The signals can be provided from the same logic circuit whichregulates the frequency of operation of inverter 183, or from anindependent logic circuit. The closure of static switch 193 placesresistor 192 across inductor 17a, so that load current primarily flowsthrough resistor 192 and capacitor 16a. The on time of the switch isgradually increased, as shown in the successive half-cycles of thewaveform 204 in FIG. 7B. The static switch is no longer in the circuitafter the load current reaches rated value. This arrangement insuresthat very little transient disturbance appears at conductors 198, 199 asthe load is initially applied.

In the reduction of load current, the circuit including static switch200 is utilized. In this arrangement the lower connection of the staticswitch is coupled over a terminal 205 to terminal 190. This staticswitch is then time-modulated as already explained in connection withFIG. 7B. It is also apparent, from the previous explanation of the useof a clamping transformer (see, for example, FIG. 68), that terminals186 and 190 can be clamped back to the DC bus to provide another measureof regulation.

An alternative connection for static switch 200 is represented by thedashed line 206. That is, the connection between terminals 205 and190'can be interrupted, and the connection from terminal 205 extended upto terminal 187. This affords current control across capacitor 16aduring load application analogous to that described in connection withstatic switch 193 and inductor 17a.

FIG. 7C illustrates an alternate method of minimizing the transienteffect during load application and subsequent removal of the load. Oneoutput line 184 from inverter 183 is coupled through inductor 17a andcapacitor 16a in the series filter. The other line 188'is connecteddirectly to output conductor 199. A secondary winding 210 ismagnetically coupled to inductor 17a to provide a transformer assembly,in which winding 210 has a pair of end connections and a center tapconnection. A pair of SCRs 211, 212 are coupled between the respectiveend connections of winding 210 and DC bus conductor 12a. The center tapconnection of winding 210 is coupled directly to DC bus conductor 12b.Transformer. 62, 65 provides a similar coupling to the DC bus conductorsfrom capacitor 16a, over a circuit including SCRs 214, 215 and conductor216.

Utilizing the arrangement of FIG. 7C, gradual application of the load isaccomplished by programming or gradually modifying the on times of SCRs211, 212 as explained above in connection with FIGS. 7A and 78. Loadremoval is accomplished in a similar manner, by regulating the times ofconduction and non-conduction ofthe SCRs 214, 215.

It is manifest that the protection of the energy transfer system can beachieved either from a signal indicating internal component failure,such as a fuse blowing in the inverter or some associated circuit, or anovercurrent condition can be sensed as described generally in connectionwith FIG. 2. By placing a virtual short circuit across either of the twoseries-coupled reactive components in the filter, the other filtercomponent limits the transfer of load current under the fault condition.This is done very rapidly, without any physical movement by completelystatic components. The subsequent physical interruption of the energytransfer circuit, as by the operation of the relay including winding107, is a refinement of the invention. By using only the transformer anddiodes clamped back to the DC bus, load current has been reduced below80 percent of the normal or rated current by detuning the filter. Withmultiple systems, as shown in FIG. 5, use of the invention decouples thesystem with the fault from the still operating systems. Were this notdone, the remaining systems would be severely damaged as they attemptedto supply the fault current. Protection for the inverter-load circuitcan also be. achieved by coupling the protective circuits in aline-to-line arrangement, with the connections being made to the commonterminals between the reactive filter components. Use of static switchesacross the reactive filter components, and programming these switchesduring load application and load removal, minimizes transient effects onthe system.

While only particular embodiments of the invention have been describedand illustrated, it is manifest that various modifications andalterations may be made therein. It is therefore the intention in theappended claims to cover all such modifications and alterations as mayfall within the true spirit and scope of the invention.

What is claimed is:

1. An energizing system for passing AC energy from an inverter over afilter which includes a capacitor coupled in series with an inductor,the reactances of the capacitor and inductor being substantially equalat the normal frequency of the inverter AC output voltage, including theimprovement which comprises:

coupling means, having a first portion coupled in parallel with one ofthe reactive components of the filter, and having a second portionmagnetically coupled to the first portion;

a static switch, including a reference impedance of very low value, andat least one semiconductor switch, connected such that conduction of thesemiconductor switch effectively couples the reference impedance overthe coupling means to substantially short out the filter componentcoupled in parallel with the first portion of the coupling means; and afiring circuit, connected to provide a control signal for turning on thesemiconductor switch upon detection of an overload condition associatedwith the inverter. 2. An energizing system having an inverter energizedfrom a DC bus, the inverter passing AC energy over a filter whichincludes a capacitor series-coupled with an inductor, with thereactances of the capacitor and inductor being substantially equal atthe normal frequency of the inverter AC output voltage, including theimprovement which comprises:

a transformer, having a first winding coupled across the capacitor, asecond winding magnetically coupled to the first winding, and a centertap connection on the second winding;

means including a rectifier for coupling the DC bus to the secondwinding of the transformer, to clamp the voltage across the capacitor atthe DC bus level when the capacitor voltage tends to increase;

a static switch, including a diode coupled in series with asemiconductor switch between a pair of reference conductors, a lowimpedance coupled between the same reference conductors, and means forcoupling the center tap connection of the second transformer winding tothe common connection between the diode and the semiconductor switch,such that conduction of the semiconductor switch couples the lowimpedance across the transformer to shunt the capacitor in the filter;and firing circuit, including a second semiconductor switch connected tobe gated on when a signal denoting an overload condition is received bythe firing circuit, and means for providing a supply voltage in thefiring circuit, such that conduction of the second semiconductor switchpasses the supply voltage to gate on the semiconductor switch in thestatic switch, effectively de-tuning the filter and reducing the ACcurrent flow from the inverter.

3. An energizing system as claimed in claim 2 in which the firingcircuit further comprises a relay, connected for actuation upon firingof the second semiconductor switch, to interrupt the energy transfercircuit between the filter and an AC load at a predetermined time afterfiring of the semiconductor switch in the static switch.

1. An energizing system for passing AC energy from an inverter over afilter which includes a capacitor coupled in series with an inductor,the reactances of the capacitor and inductor being substantially equalat the normal frequency of the inverter AC output voltage, including theimprovement which comprises: coupling means, having a first portioncoupled in parallel with one of the reactive components of the filter,and having a second portion magnetically coupled to the first portion; astatic switch, including a reference impedance of very low value, and atleast one semiconductor switch, connected such that conduction of thesemiconductor switch effectively couples the reference impedance overthe coupling means to substantially short out the filter componentcoupled in parallel with the first portion of the coupling means; and afiring circuit, connected to provide a control signal for turning on thesemiconductor switch upon detection of an overload condition associatedwith the inverter.
 2. An energizing system having an inverter energizedfrom a DC bus, the inverter passing AC energy over a filter whichincludes a capacitor series-coupled with an inductor, with thereactances of the capacitor aNd inductor being substantially equal atthe normal frequency of the inverter AC output voltage, including theimprovement which comprises: a transformer, having a first windingcoupled across the capacitor, a second winding magnetically coupled tothe first winding, and a center tap connection on the second winding;means including a rectifier for coupling the DC bus to the secondwinding of the transformer, to clamp the voltage across the capacitor atthe DC bus level when the capacitor voltage tends to increase; a staticswitch, including a diode coupled in series with a semiconductor switchbetween a pair of reference conductors, a low impedance coupled betweenthe same reference conductors, and means for coupling the center tapconnection of the second transformer winding to the common connectionbetween the diode and the semiconductor switch, such that conduction ofthe semiconductor switch couples the low impedance across thetransformer to shunt the capacitor in the filter; and a firing circuit,including a second semiconductor switch connected to be gated on when asignal denoting an overload condition is received by the firing circuit,and means for providing a supply voltage in the firing circuit, suchthat conduction of the second semiconductor switch passes the supplyvoltage to gate on the semiconductor switch in the static switch,effectively de-tuning the filter and reducing the AC current flow fromthe inverter.
 3. An energizing system as claimed in claim 2 in which thefiring circuit further comprises a relay, connected for actuation uponfiring of the second semiconductor switch, to interrupt the energytransfer circuit between the filter and an AC load at a predeterminedtime after firing of the semiconductor switch in the static switch.